In the early days of the telephone art, use of the telephone was often confined to communications among users within a local geographic area. As a result and over the years, the economies related to accessing a communications system have lead to telephones in a local area usually being interconnected through a central controller, often called a local central office in the art.
As digital computers came upon the scene, another local community of use was discernible. Hence, a central controller is commonly employed for interconnecting various user terminals. For example, U.S. Pat. No. 3,851,104; entitled "Digital Communications System" and issued Nov. 26, 1974; discloses a time division multiple access communications system which, under the control of a central terminal, provides communication links among a plurality of user terminals by way of a single communications signal path.
As the digital computer art advanced, parallel advances in the semiconductor art have lead to smaller, relatively inexpensive computers, commonly referred to as microprocessors. With the advent of such smaller computers, the technique of central control is being abandoned in favor of a distributed control technique which allows the distributed microprocessors to exercise more sophisticated communication control. Also, because of the usually bursty nature of digital information, the recent trend has also been toward communications systems having a capability for handling packets of digital information. One such distributed control communications system is disclosed in U.S. Pat. No. 4,063,220; entitled "Multipoint Data Communication System with Collision Detection" and issued Dec. 13, 1977. Indeed, U.S. Pat. No. 4,063,220 discloses a communications system in which, when a terminal is to start an intended transmission on a communications path, a phase decoder detects the presence of other transmissions on the path and, responsive thereto, delays the intended transmission until no other transmissions are sensed. Once a transmission has started, if an interference (or collision) therewith is detected, a random number generator is used to select an interval of time, at the completion of which a retransmission of the packet will again be attempted.
The single path of U.S. Pat. No. 4,063,220 is shared by all terminals (or stations or sites). Since all terminals share the single path, it is possible to transmit a message from one terminal to a plurality of terminals. Such a message is characteristically called a broadcast message. It should come as no surprise then that the art is looking for a reliable transmission protocol which can assure that all terminals, which are intended to receive a broadcast message, do indeed receive the broadcase message.